Dual-position calibration markers for total hip arthroplasty: theoretical comparison to fixed calibration and single marker method.

Digital templating is considered a standard for total hip arthroplasty. Different means for the necessary calibration of radiographs are known. While single marker calibration with radiopaque spheres is the most common, it is associated with possible significant deviations from the true magnification of the hip. Notably, fixed magnification factors showed better results. Therefore, a dual-position calibration marker method was simulated and compared to the established methods. First, an empirical fixed magnification factor was identified and applied to a series of radiographs. Second, three magnification factors were generated based on sagittal patient data of 398 CT scans. These methods were compared to the fixed factor. The fixed factor was 122.6%. In the clinical application, the error of the fixed factor was 2.5% while the error of the single marker was 5.2%. In the CT cohort, the mean reference factor was 120.5% in females and 120.3% in males. The reference factor was compared to sex-specific means, sex-specific linear functions, and sex-specific cubic functions. The best results were found for the linear regression model with a mean difference of 0.8% from the reference value. No proportional bias was found (p = 0.623). The simulation of the dual-position marker method using the linear regression model showed promising results, superior to all other methods. In future studies, its clinical application should be tested.